1. Field of the Invention
The present invention relates to pulse doppler or moving target indicator (MTI) radar systems; and more particularly, to an improved digital filter bank signal processor utilizing variable interpulse periods and weighting.
2. Description of the Prior Art
Moving target indication (MTI) radar is provided to reject signals or echoes from stationary and slowly moving objects, such as terrain, foliage or surface vehicles; and to pass echoes from moving objects such as aircraft. The radar receivers may utilize digital filters to suppress such undesired echoes; and these filters are generally described as moving target indicators. The MTI signal processor utilizes the doppler shift caused by the reflected signal of a moving target to distinguish moving targets from fixed targets. In a pulse-radar system this doppler shift appears as a change of phase of received signals between consecutive radar pulses. Assuming that the radar transmits a pulse of RF energy, which is reflected by ground clutter and a moving target such as an airplane, the reflected pulses return to the radar antenna within a certain length of time. The radar then transmits a second pulse. The reflection from the ground clutter occurs in exactly the same amount of time for both the first and second transmitted pulses, but the reflection from the aircraft occurs in more or less time, because the aircraft has moved either closer to or away from the radar in the interval between transmitted pulses. The time change between the first and second transmitted pulses is determined by comparing the phase of the received signal with the phase of the reference oscillator in the radar. If the target is fixed the relative phase of consecutive received pulses does not change. For a target that moves between pulses, the phase of the received pulses change.
In the event of both wind and rain, such moving rain may be detected as a moving target rather than clutter. Wind conditions vary as a function of altitude, a condition known as "wind shear", so rain echoes cover a band of velocities. Particularly, when the radar antenna is scanning and is pointed either directly into the wind or with the wind, the rain clutter will present the greatest radial velocity relative to the radar, and this could be in the order of 40-60 knots. Inasmuch as such a low velocity does not often exist in the detection of flying aircraft, the system can be so constructed to reject any clutter or interference that has a radial velocity equal to that of the rain. A flying aircraft can create such low radial velocity when the aircraft is flying nearly tengentially relative to the antenna.
In the past, such systems have been constructed either as single channel filtering systems, generally known as MTI circuits, or as multiple channel filter systems, recently given the name of MTD circuits. In the single channel or MTI approach it is necessary that the clutter rejection filters be designed to reject clutter at all possible velocities simultaneously; for example, the filter rejection notch might need to extend from -50 knots to +50 knots in order to cope with any possible wind condition in the case of rain clutter, even though the actual rain clutter present at any one instant, corresponding to a particular antenna pointing direction, would be unlikely to extend over the entire notch region. To avoid this restriction, the multiple channel or MTD approach may be employed to provide a system of filters which is adaptive to the actual clutter conditions present at any instant. Thus, for example, a bank of filters may be used, which in aggregate cover the velocity range -50 to +50 knots but each of which has a narrower velocity coverage over a small part of that velocity range. Each filter in the bank may then be equipped with Constant False Alarm Rate (CFAR) circuits, of a conventional nature, at its output, such that in the presence of interfering clutter, such as rain clutter, the particular filters, into which the rain echoes fall, are desensitized by their CFAR circuits to the extent necessary to prevent detection of the rain clutter, whereas the remaining filters, in which rain clutter echoes are not present, retain their full sensitivity to detect aircraft targets. Thus, the CFAR circuits of the multiple filter approach enable the detection system to respond adaptively to a clutter interference environment which is changing with time, as a result, for example, of the effects of antenna scanning.
It may be noted that the conventional CFAR circuits referred to above for the individual filter outputs may be implemented in a variety of alternative ways; for example, suitable well known CFAR methods are: cell-averaging CFAR, log CFAR, or hard-limiting types of CFAR such as CPACS (Coded Pulse Anti-Clutter System).
There are times, when the received signal is shifted precisely 360.degree., or multiplies thereof, between pulses. Such as the case, when the targets move 1/2, 1, 3/2, etc. wavelengths between consecutive transmitted pulses. Thus, where the radar system is so structured to provide a zero output not only for stationary targets or clutter but also from targets up to 50 knots for example, to reject wind blown rain, such problem is aggravated. Because not only are the multiples of 360.degree. phase shift rejected, but also a band of phase shifts adjacent to the multiples corresponding to the wind and rain clutter for a particular area. This rejection of the frequency multiples which are echoed from a moving target are known as "blind speeds". Thus, blind speeds represent the frequency ambiguity inherent in a sample data system when the interval between data samples (interpulse period) is fixed. The echoes generated by an object moving an integer number of half-wavelengths toward or away from the radar antenna during the interpulse period are indistinguishable from those of a stationary object. Therefore, if ground clutter interferences are rejected by the filter bank, the system also is blind to aircraft speeds which create these ambiguous doppler frequencies.
Heretofore, filters for such radars were implemented with analog devices such as capacitors, inductors and resistors. However, more recently digital filters have been utilized primarily because of lower cost of implementation when a large number of range cells must be covered. In both the analog and digital implementations, the echoes of the radar receiver are sampled at an interval equal to or less than the range resolution of the radar. Successive radar transmissions provide a multiplicity of samples for each of range cell of interest, which create the inputs for a bank of filters at each point in range.
Most digital processors or filters utilize the Discrete Fourier Transform mathematical operation to convert time separated data inputs into frequency dependent data outputs. Although the Fast Fourier Transform is a practical configuration which reduces the number of mathematical operations which must be performed, it requires the data input be collected at a fixed interpulsed period, which does not eliminate the "blind speed" deficiency. Also, analog filters suffer from the same blind speed deficiency; in that they do not provide the desired rejection of interference frequencies if the interpulse period is variable; and of course, a fixed interpulse period creates blind speeds.
One of the virtues of digital implementation of MTI filters is the ability to quickly shift from one pulse repetition frequency to another so that a target that is blind to one pulse repetition frequency (PRF) is visible on another. Unfortunately, desired azimuth beam widths and scan rates of the antenna generally do not provide an adequate number of echoes as the beam scans across the target for this solution to be effective.
The previously mentioned MTD system, implemented for an airport surveillance radar operating at a frequency of 3 GHz., employs two interpulse periods: a burst of ten pulses having a minimal interpulse period for the desired range coverage, followed by a second burst of ten pulses with a 25% longer interpulse period. The combination of azimuth beam widths, scan rate, and PRF provide 23 hits per beam width, between -6 dB points of echo amplitude, which is barely enough for the use of two different PRF's. The 25% spread of PRF's is the maximum tolerable, which creates a first blind speed of approximately 560 knots over ground clutter interference. Thus the system has a modest range such as 58 nmi, for example, and a velocity coverage of approximately 500 knots. Such a proposed system provides this coverage most effectively when the only interference is ground clutter. However, when simultaneous rain and ground clutter interference occur, severe degradation of sensitivity results at certain aircraft velocities (dim speeds). Referring to FIG. 1 as an example, the velocity response of such a proposed system with two pulse repetition frequencies in a combination of ground clutter and a particular case of rain clutter, is shown. In this example the velocity spectrum of the rain 20 is chosen to extend from approximately 15 to 55 knots. The portions of the curve that are cross hatched illustrate aircraft velocities which are processed with good sensitivity. However, between such cross hatched portions of the curve are velocities where sensitivity is seriously degraded, referred to at 10, 11, 12, 13 and 14 as well as at 15, 16, 17, 18 and 19. These notches represent the result of the system having adapted to the rain spectrum designated at 20, which is rejected by the deep clutter notch at 10. The other notches, 11 through 19, are not desired. Under these conditions, as shown in FIG. 1 the following dim speeds correspond to the clutter notches 11 through 19 as follows:
______________________________________ Notch Number Dim Speed Region ______________________________________ 11 125 to 181 knots 12 288 to 300 knots 13 350 to 362 knots 14 456 to 500 knots 15 -75 to -125 knots 16 -231 to -244 knots 17 -294 to -306 knots 18 -400 to -437 knots 19 -525 to -600 knots ______________________________________
Thus, it is desirable to provide an MTI system that provides detection of all aircraft velocities of interest, except those velocities close to the velocities of rain, chaff or ground clutter without severe degradation at doppler frequencies which are multiples of the PRF; or in other words, without blind speeds.